Process for preparing ambient temperature ionic liquids

ABSTRACT

A process for preparing an ionic liquid or salt, preferably in which the cation comprises an N-alkylated base and the anion is a carboxylate, formed by reaction between an organic base and an alkylating agent, wherein the alkylating agent is a fluorinated ester or an alkyl sulfonate, is described. Suitable organic bases include imizadoles, substituted imidazoles, pyridines and substituted pyridines. The so-formed products can be subsequently transformed into different ionic liquids or salts by metathesis.

[0001] This invention relates to a process for processing ambienttemperature ionic liquids.

[0002] Ambient temperature ionic liquids based upon the1,3-dialkylimidazolium cation were first reported in 1982 by Wilkes etal¹. These systems were based upon the chloroaluminate anion andalthough they possess many useful properties (e.g. wide liquids, thermalstability and large electrochemical window) they are reactive to certainmaterials and are sensitive to moisture. An air and water stable systemwas developed by Wilkes and Zaworotko in 1992 based upon thetetrafluoroborate anion². Since this report a wide range of ionicliquids containing different anions have appeared in the literature³.These systems have received much attention and recent studies have shownthat ambient temperature ionic liquids can be used as solvents for arange of chemical reactions including polymerisation⁴, hydrogenation⁵,Friedel-Crafts acylations⁶ and for the Diels-Alder reaction⁷.

[0003] The principal route currently employed in the synthesis of theair and moisture stable 1,3-dialkylimidazolium ionic liquids is outlinedin Scheme 1.

[0004] The first step with this method is the alkylation of1-alkylimidazole with a haloalkane to give a 1,3-dialkylimidazoliumhalide salt. The second step is metathesis of the halide for theappropriate anion. The second step can be carried out with either anacid or a metal salt to eliminate H-Hal as or precipitate M⁺Halrespectively. It is here that the intrinsically good solvatingproperties of these ionic liquids become a problem. In many of thesyntheses the ionic liquids solvate the halide waste so effectively thatcomplete removal is not effected. Halide contamination of the ionicliquids is a problem that must be overcome for them to be used asreaction solvents on a large scale. For instance, when used as media fortransition metal catalysed reactions the presence of stronglyco-ordinating halide ions have been shown to reduce catalyst activity⁵.The opportunity exists in many reactions for the residual halides to beoxidised to halogens which will result with many substrates and cancorrode apparatus. In addition, this method always generates astoicheiometric amount of halide salt as a waste product. Whenmetathesis is carried out using a silver salt the route becomesprohibitively expensive upon scale up. Employing the alkali metal saltsreduces the cost, but not the waste.

[0005] We have developed a new method for the synthesis of the air- andmoisture-stable ionic liquids that overcomes the possibility of halideimpurities and reduces the amount of waste products. This method isbased upon the use of fluorinated esters or alkyl sulfonates asreplacements for haloalkanes.

[0006] Thus, according to one aspect of the present invention, there isprovided a process for preparing an ionic liquid or salt formed byreaction between an organic base and an alkylating agent, wherein thealkylating agent is a fluorinated ester or an alkyl sulfonate.

[0007] The so-formed product of the organic base and ester or sulfonatecould subsequently be transformed into a different ionic liquid or saltwith a range of different anions by metathesis, preferably using an acidor metal salt.

[0008] In one embodiment of the present invention, the cation formed isan N-alkylated base.

[0009] For this, the organic base could be an imidazole or a substitutedimidazole. Preferably, the substituted imidazolium salt is a1,3-dialkylimidazolium trifluoroethanoate and the (n-1)-substitutedimidazole is a 1-alkylimidazole.

[0010] Alternatively, the organic base is a pyridine or a substitutedpyridine.

[0011] Other organic bases include the phosphines and sulfides.

[0012] Also preferably a co-solvent is used.

[0013] The following description will focus on using the organic base1-methylimidazole, the imidazole most commonly used in the preparationof ambient temperature ionic liquids, and ethyl trifluoroethanoate asthe alkylating agent.

[0014] The synthesis is similar to that mentioned above in Scheme 1, inthat there is an alkylation and a metathesis step to give the desiredionic liquid as shown in Scheme 2.

[0015] The reaction of 1-methylimidazole with ethyl trifluoroethanoateto give 1-ethyl-3-methylimidazolium trifluoroethanoate, [emim] [TFA],proceeds cleanly and smoothly at moderate temperature (70° C.). However,some reduction in the rate of reaction may occur as the reactionproceeds. The primary reason for the reduction in rate is that unreacted1-methylimidazole concentrates in the ionic liquid phase as it forms,while the ethyl trifluoroethanoate is only slightly soluble in [emim][TFA]; thus reactants are kept apart Addition of a co-solvent tosolubilise reactants and products, for example acetonitrile, overcomesthis problem and a significant rate enhancement is observed.Alternatively, the reaction may be performed in an autoclave.

[0016] [emim] [TFA] is an ambient temperature ionic liquid with all theexpected characteristics in its own right. In addition, it is a goodstarting point for the synthesis of other air- and moisture-stable ionicliquids with metathesis of the trifluoroethanoate anion easily achieved.Addition of the desired acid to [emim] [TFA] yields a reaction mixturewith only one volatile material, trifluoroethanoic acid (b.pt.72° C.),which is easily removed under vacuum. This is true as long as the addedacid is of higher boiling point than CF₃CO₂H, which most acids ofinterest are (e.g. HPF₆, HBF₄, H₃PM₁₂O₄₀ (M=W, Mo), H₃PO₄). This givesthe desired ionic liquid, without extractions and washings, in a halidefree state.

[0017] The use of longer alkyl chain esters (e.g. hexyltrifluoroethanoate) works equally as well with 1-alkylimidazoles to givethe desired product. The use of more fluorinated esters (e.g. ethylheptafluorobutanoate) is still possible although they may have thedrawback of generating a less volatile carboxylic acid by-product.

[0018] Alkyl sulfonates for use as the alkylating agent are also wellknown in the art, such as a methyl sulfonate; more particularly butylmethylsulfonate.

[0019] According to a second aspect of the present invention there isprovided a process for preparing an ionic liquid or salt formed byreaction between an organic base and fluorinated alkylating agentwhenever the so-formed fluorinated by-product has a lower boiling pointthan the acid added to the alkylating agent.

[0020] The cation formed is preferably an N-alkylated base. This is ageneral method that can be used to synthesise a range of (imidazolium,possibly substituted imidazolium) ionic liquids and low melting pointsalts.

[0021] The present invention extends to any product obtainable from anyof the new processes herein described. Particularly, it extends to a1,3-dialkylimidazolium-based ionic liquid whenever prepared by reacting1-alkylimidazole with a fluorinated ester, followed by metathesis.

[0022] The present invention also extends to the use of any ester ableto act in a similar manner to form an ambient temperature ionic liquidwith an organic base.

[0023] The reaction conditions required to effect the processes of thepresent invention will be known or calculable to those skilled in theart.

[0024] The use of fluorinated compounds, although expensive, is desiredfor two reasons. Firstly, fluorination of the ester activates themolecule for the alkylation step, and secondly, fluorinated products aremore volatile and of lower boiling point than their non-fluorinatedanalogues, thus making separation of the ionic liquid easier. The costof using fluorinated esters should not be prohibitively expensive as thecarboxylic acid by-product can be recycled. An overall process isenvisaged as shown in Scheme 3.

[0025] R=hydrocarbyl, or substituted hydrocarbyl.

[0026] X=any anion such as nitrate, tetrafluoroborate,hexafluorophosphate, etc.

[0027] mim=1-methylimidazole.

[0028] R and X are used in their normal context as is well known in theart.

[0029] As scheme 3 shows, the waste trifluoroethanoic acid is recoveredand converted into the reactive ester either through a straightesterification or via the anhydride. This gives the following balancedequation for the synthesis of ambient temperature ionic liquids;

mim+ROH+HX→[Rmim] [X]+H₂O

[0030] The present invention thus provides a new synthetic route toambient temperature ionic liquids that ensures the product ishalide-free. If the metathesis is performed with an acid rather than ametal salt, then the product will be both halide-free and metal-free. Inaddition, the alkylating agent can be regenerated from inexpensive andreadily available materials, thus reducing waste.

EXPERIMENTAL Preparation of 1-ethyl-3-methylimidazoliumtrifluoroethanoate, [emim] [TFA]

[0031] 1-Methylimidazole (2.5 g, 30.4 mmol) and ethyl trifluoroethanoate(25.8 g, 181.6 mmol) were dissolved in ethanenitrile (20 cm³). Theresultant solution was placed in a sealed glass vessel and stirred at70° C. for 5 days giving a pale yellow solution. The volatiles wereremoved in vacuo giving [emim] [TFA] in 100% yield.

Preparation of 1-ethyl-3-methylimidazolium tetrafluoroborate, [emim][BF₄]

[0032] To [emim] [TFA] (1.0 g, 4.5 mmol) was added one equivalent offluoroboric acid (0.412 cm³ of 10.8M aq. solution, 4.5 mmol) and themixture was stirred overnight at room temperature. Heating under vacuumat 100° C. removes trifluoroethanoic acid and water giving [emim] [BF₄].

Preparation of 1-ethyl-3-methylimidazolium hexafluorophosphate, [emim][PF₆]

[0033] To [emim] [TFA] (2.0 g, 8.9 mmol) dissolved in water (10 cm³) wasadded hexafluorophosphoric acid (2 cm³ of 6.79M aq. solution, 13.58mmol). This gave [emim] [PF₆] as a white precipitate which was collectedby vacuum filtration.

Preparation of butyl methanesulfonate (BuOMs)

[0034] To a 500 cm³ round-bottomed flask, equipped with a magneticstirrer and pressure equalising dropping funnel, was added butanol (55,6g, 0.75 mol), triethylamine (55.7 g, 0.55 mol) and dichloromethane (300cm³). Methanesulfonyl chloride (57.3 g, 0.05 mol) was then addeddropwise over a two-hour period from the dropping funnel, with coolingfrom an ice bath. The mixture was stirred for a further 24 hours at roomtemperature. The reaction mixture was filtered, concentrated on a rotaryevaporator, and distilled (bp−80-90 ° C. at 5 mm Hg). This gave 68.1 g(98%) of a colourless oil.

Preparation of 1-butyl-3-methylimidazolium methanesulfonate ([bmim][Oms])

[0035] In a 100 cm³ round-bottomed flask, was added butylmethanesulfonate (15.3 g, 0.10 mol) and 1-methylimidazole (8.21 g, 0.10mol). A reflux condenser was attached and the mixture heated at 100° C.for 48 hours. A vacuum was applied to the flask (1 mm Hg) to removeunreacted starting materials for 12 hours at 80° C. The low-melting salt[bmim] [Oms] (22.3 g, 95%) solidified on cooling.

REFERENCES

[0036] 1. J. S. Wilkes, J. A. Levisky, R. A. Wilson and C. L. Hussey,Inorg. Chem., 1982, 21, 1263.

[0037] 2. J. S., Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem.Commun., 1992, 965.

[0038] 3. C. M. Gordon, J. Holbrey, A. R. Kennedy and K. R. Seddon,J.Mater. Chem., 1998, 1, 2627; E. I. Cooper and E. J. M. O'Sullivan, inMolten Salts, Eds. R. J. Gale, G. Blomgren and H. Kojima, TheElectrochemical Society Proceedings Series, Pennington, N.J., 1992, 16,386; P. Bonhote, A. P. Diaz, N. Papageorgiou, K. Kalanasundaram and M.Gratzel, Inorg. Chem., 1996, 35, 1168; M. Fields, F. V. Hutson, K. R.Seddon and C. M. Gordon, World Patent, WO 98/06106, 1998.

[0039] 4. A. A. K. Abdul-Sada, P. W. Ambler, P. K. G. Hodgson, K. R.Seddon and N. J. Stewart, World Patent, WO 95/21871, 1995.

[0040] 5. Y. Chauvin, L. Mussmann and H. Olivier, Angew. Chem. Int. Ed.Engl., 1995, 34, 2698; P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R.F. de Souza and J. Dupont, Polyhedron, 1996, 1217; A. L. Monteiro, F. K.Zinn, R. F. de Souza and J. Dupont, Tetrahedron-Asymmetry, 1997, 8, 177;P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J.Dupont, Inorg. Chim. Acta, 1997, 255, 207.

[0041] 6. C. J. Adams, M. J. Earle, G. Roberts and K. R. Seddon, Chem.Commum., 1998, 2097; J. A. Boon, J. A. Levisky, J. L. Pflug and J. S.Wilkes, J. Org. Chem. 1986, 51, 480.

[0042] 7. M. J. Earle, P. B. McCormac., and K. R. Seddon, Green Chem.1999, 1, 23.

1. A process for preparing an ionic liquid or salt formed by reactionbetween an organic base and an alkylating agent, wherein the alkylatingagent is a fluorinated ester or an alkyl sulfonate.
 2. A process asclaimed in claim 1 wherein the cation formed is an N-alkylated base. 3.A process as claimed in claim 2 wherein the organic base is an imidazoleor a substituted imidazole.
 4. A process as claimed in claim 3 whereinthe organic base is a 1-alkylimidazole.
 5. A process as claimed in claim4 wherein the organic base is 1-methylimidazole.
 6. A process as claimedin claim 2 wherein the organic base is a pyridine or a substitutedpyridine.
 7. A process as claimed in claim 6 wherein the organic base isan alkylpyridine.
 8. A process as claimed in claim 1 wherein the organicbase is a phosphine or a sulphide
 9. A process as claimed in any one ofthe preceding claims wherein a co-solvent is used.
 10. A process asclaimed in claim 9 wherein the co-solvent is acetonitrile.
 11. A processas claimed in any one of the preceding claims wherein the reaction iscarried out under pressure.
 12. A process as claimed in any one of thepreceding claims wherein the anion formed is trifluoroethanoate.
 13. Aprocess as claimed in any one of the preceding claims wherein thealkylating agent is ethyl trifluoroethanoate.
 14. A process as claimedin any one of claims 1-12 wherein the alkylating agent is a methylsulfonate.
 15. A process as claimed in claim 14 wherein the alkylatingagent is butyl methylsulfonate.
 16. A process as claimed in any one ofthe preceding claims wherein the so-formed product is subsequentlytransformed into a different ionic liquid or salt by metathesis.
 17. Aprocess as claimed in claim 16 wherein an acid or metal salt is used forthe metathesis.
 18. A process for preparing an ionic liquid or saltformed by reaction between an organic base and fluorinated alkylatingagent whenever the so-formed fluorinated by-product has a lower boilingpoint than the acid added to the alkylating agent.
 19. An ionic liquidor salt whenever prepared by a process as claimed in claims 1-18.
 20. A1, 3-dialkylimidazolium trifluoroethanoate whenever prepared by aprocess as claimed in any one of claims 1-18.